The present invention relates to a microwave dielectric ceramic composition employed in resonator materials or capacitor materials etc used in the microwave region of a few GHz.
Dielectrics are used in the resonators, filters, or capacitors that are used in transceivers for for example satellite communication broadcasting or mobile identification devices using microwaves of a few GHz.
As such dielectric ceramic materials, for example BaOxe2x80x94TiO2xe2x80x94Nd2O3xe2x80x94Bi2O3 compositions have been proposed (Laid-open Japanese Patent Application No. Sho. 61-8806). The dielectric constant ∈ of this composition is of the order or 70 to 110; when dielectric resonators or capacitors are constructed, materials of larger dielectric constant ∈ are preferred since the larger the dielectric constant ∈ of the material that is used, the smaller are the dimensions of the resonator.
Conventional materials of large dielectric constant ∈ include for example SrTiO3 and CaTiO3 etc; while their dielectric constant ∈ is very large at 300 and 180, the temperature coefficient xcfx84f of the resonance frequency is extremely large at +1700 ppm/xc2x0C. and +800 ppm/xc2x0C., which means that it is not possible to use them.
Accordingly, as a method of lowering the temperature coefficient xcfx84f of a dielectric ceramic composition, the method is available of preparing a material whose dielectric constant ∈ is as large as possible and whose temperature coefficient xcfx84f has a negative value; with this method, a ceramic composition can be obtained by a suitable composition whose dielectric constant ∈ is large and whose temperature coefficient xcfx84f of resonance frequency is small.
For example, Laid-open Japanese Patent Publication No. H. 5-211009 proposes the obtaining of a ceramic composition of large dielectric constant ∈ and whose temperature coefficient of resonance frequency xcfx84f is close to zero, by preparing a material represented by the compositional formula (A1+1/2.B3+1/2) TiO3 where A1+ is Li1+, and B3+ is Nd3+, Sm3+, Co3+ or Pr3+, constituting a material of large dielectric constant ∈ and wherein the temperature coefficient xcfx84f has a negative value.
However, the demands for miniaturization of portable electronic terminal equipment have today become so exacting that a material having even higher dielectric constant ∈ is earnestly sought for the dielectric material of resonators, filters and capacitors employed in such devices.
In view of the characteristics required for a dielectric ceramic composition for microwaves, an object of the present invention is to obtain a dielectric ceramic composition wherein the dielectric constant ∈ is large, the temperature coefficient xcfx84f of the resonance frequency is close to 0, and which has a large Q value, by adding to and blending with a ceramic composition whose xcfx84f of the resonance frequency is large on the plus side to a ceramic composition whose temperature coefficient xcfx84f is large on the minus side.
With the object of providing a dielectric ceramic composition whose dielectric constant ∈ is large, whose temperature coefficient of resonance frequency xcfx84f is close to 0, and whose Q value is large, the present inventors have previously proposed a dielectric ceramic composition represented by the compositional formula Li2Oxe2x80x94Na2Oxe2x80x94Bi2O3xe2x80x94R2O3xe2x80x94TiO2, where R2O3 is one or two or more of La2O3, Nd2O3, Sm2O3, Co2O3 or Pr2O3 (Japanese Patent Application Number 2000-337141).
Also, the present inventors have proposed (Japanese Patent Application Number 2000-340104) a dielectric ceramic composition represented by the compositional formula Li2Oxe2x80x94Bi2O3xe2x80x94R2O3xe2x80x94SrTiO3, where R2O3 is one or two or more of La2O3, Nd2O3, Sm2O3, Co2O3 or Pr2O3.
As a result of further assiduous investigation, the present inventors discovered that an excellent dielectric characteristic could be obtained in stable fashion by co-presence of specific quantities of Bi2O3, Na2O and MO (where M is one or two of Ca and Sr) in the one or two or more of La, Nd, Pr and Sm constituting R, in the Li2Oxe2x80x94R2O3xe2x80x94TiO2-based composition.
Specifically, the present inventors discovered that, in an Li2Oxe2x80x94R2O3xe2x80x94TiO2-based composition, an improved dielectric constant ∈ could be achieved by introducing a specific quantity of Bi2O3, and xcfx84f could be shifted to the plus side and in addition the effect of a considerable improvement in Qf achieved by introducing a specific quantity of MO (where M is one or two of Ca and Sr).
They discovered that, by introducing MO, xcfx84f becomes too large, making it difficult to make xcfx84f approach 0 while maintaining Qf at a fixed value, so, by introducing a specific quantity of Na2O together with the MO (where M is one or two of Ca and Sr), in particular in the case of material of ∈r greater than 150, it was possible to control xcfx84f to the vicinity of 0 while maintaining Qf at an high value, and thereby perfected the present invention.
Specifically, the present invention consists in a microwave dielectric ceramic composition represented by the compositional formula
aLi2O-bNa2O-cR2O3-dBi2O3-eMO-fTiO2
(where a+b+c+d+e+f=100, a, b, c, d, e and f being mol %) where R includes one or two or more of Nd, Sm, Pr, and La, M includes one or two of Ca and Sr and a, b, c, d, e, f satisfy 5 less than a less than 15, 0 less than b less than 10, 3 less than c less than 15, 1 less than d less than 15, 1 less than e less than 30, and 40 less than f less than 75.
In the present invention, in the Li2Oxe2x80x94Na2Oxe2x80x94R2O3xe2x80x94Bi2O3xe2x80x94MOxe2x80x94TiO2-based composition (where R is one or two or more of Nd, Sm, Pr, and La and M is one or two of Ca and Sr), if Li2O is less than 5 mol %, the dielectric constant is low and xcfx84f becomes too large on the plus side; while if it exceeds 15 mol %, Li2O being of low melting-point, this is undesirable because there is the problem that the ceramic composition reacts and fuses with the base plate or base powder during sintering. A range of 5 less than a less than 15 mol % is therefore specified for Li2O (a).
Also, if Na2O exceeds 10 mol %, this is undesirable because it gives rise to the problem that xcfx84f becomes too large on the plus side, and Qf falls. Accordingly, a range of 0 less than b less than 10 mol % is specified for Na2O (b).
If R2O3 (R3+ is one or two or more of Nd3+, Sm3+, Pr3+, and La3+) is less than 3 mol %, the dielectric constant is low and Q is also poor; if it exceeds 15 mol %, there is little benefit and costs are increased, which is undesirable. A range of 3 less than c less than 15 mol % is therefore specified for R2O3 (c).
Bi2O3 has the effect of increasing the dielectric constant ∈ and a content thereof of at least 1 mol % is necessary; however, because of its lower melting-point, if it exceeds 15 mol %, there is a risk that the ceramic composition will react and fuse with the base plate or base powder on sintering; this is therefore undesirable. Accordingly, a range of 1 less than d less than 15 mol % is specified for Bi2O3 (d).
Also, the content of MO (where M is one or two of Ca and Sr) has the benefit of improving Qf and improving the temperature coefficient xcfx84f; however, if it is less than 1 mol %, there is little benefit in terms of improving Qf and improving the temperature coefficient xcfx84f and if it exceeds 30 mol %, the temperature coefficient xcfx84f becomes too large on the plus side, which is undesirable. Accordingly, a range of 1 less than e less than 30 mol % is specified for MO (e).
In addition, if TiO2 is less than 40 mol %, the required crystalline phase is not obtained and the required dielectric characteristic is not obtained; if it is more than 75 mol %, the problem arises that a phase consisting of TiO2 on its own appears, severely lowering Qf, which is undesirable. A range of 40 less than f less than 75 mol % is therefore specified for TiO2 (f).